Heretofore, there have been many attempts to improve conditions associated with heart problems. These have ranged from chemicals such as digitalis and diuretics to such devices as artificial valves and artificial hearts. The attempts over the years, however, do not appear to include the combined recognition of energy relationships, leakage of gas back into the ventricles, and the removal of gas as a means for improving the ability of a heart to pump liquid, that is, blood. This invention is directed toward obtaining and maintaining a stable heart condition with the present apparatus as a means for that objective.
The heart dynamics, or more specifically the dynamics of one side of the heart, have become analytically describable as a fluid flow dynamic process with an input flow, namely the return flow; an output flow; and a difference between the input flow and output flow representing the rate of change of the amount of fluid within the heart at a given time.
To relate to the present invention the fluid must be considered to contain both gas and liquid. By varying the amounts of the gas and liquid, changes in output flow can be produced. If, for example, some of the gas at the entrance to the aorta is allowed to repeatedly leak back through the aortic valve, between each pulsation the amount of gas within the heart can build up and be repeatedly pumped and leaked back.
As the amount of gas pumped becomes greater the amount of liquid pumped becomes less. If carried very far, this process leads to a significant reduction in liquid flow, and an instability type of condition follows, somewhat representative of a heart arrhythmia condition.
To some extent the above description parallels an analysis of fluid control apparatuses, such as for aircraft engine fuel control systems, as in each case, both contain input signals, flow in, flow out and the like. Also, to some extent the above parallels that of some engines, again input signals, flow in, and flow out as in a pumping action.
The heart action is basically that of a pump. Pumps work on an energy basis. When a pump is presented with a mixture of gas and liquid, the pump has a preference for operation on a minimal energy basis. Less energy is required to pump a given volume of a gas that to pump the same volume of a liquid.
Less energy is required to pump a given volume of gas through an orifice or restriction, and in the case of the heart, the orifice is the passage through a valve; both when the valve is properly open, and when the valve is damaged and supposed to be close but is actually partially open. If both a gas and a liquid are available, the heart has a tendency to preferentially pump a volume of gas; thus, decreasing the volume of liquid that can be pumped with a given amount of energy in a given amount of time.
If an outlet valve is damaged, in such a manner as to leak, there is a tendency for gas to leak back through the valve. The aortic valve, if damaged, can be such a valve. The gas can be preferentially pumped to the outlet side of the valve again and again, for a multitude of times.
The leaking of gas back through defective heart valves, the process being repeated, has as a ramification a decreasing amount of liquid, that is, blood, being pumped. This vapor/liquid ratio situation can be very bad with a defective heart just as with other forms of pumps, especially when the inlet pressure is very low even for very brief periods of time. In closed-loop systems, an improved capability in an active component, such as the heart in this case, can improve the overall performance of the loop.
An example of the aforementioned is the case in which heart performance is represented by a group of curves of pressure rise versus liquid volumetric flow rate with each curve represented by a different vapor/liquid ratio. Measurements may be in terms of weight flow rate or mass flow rate converted to liquid volumetric flow rate. Computerized regression methods are known and may be used to sort data, and perform related calculations, in such a manner as to describe the curves in terms of constant vapor/liquid ratio spaced in equal increments of vapor/liquid ratio. Curves can also be established for other parts of the closed-loop, such as the lungs. The combined oxygen absorption rate of the two lungs versus blood flow rate can be represented by a curve. In an analytical sense, there is a subtle but important factor involved in closed-loop systems herein explained. If component parts of the system, such as the heart and lungs, are described by the aforementioned curves, approximate transfer functions may be established for both open-loop and closed-loop performance. With such transfer functions it can be shown, as an approximation, that the closed-loop performance is related to the open-loop performance by a relationship of the following general form in Laplace transform notation: ##EQU1## note that dividing numerator and denominator by KG(s) gives: ##EQU2##
The normal heart, as a component, represents substantial gain, in terms of power ratio, pressure ratio, and flow ratio. This is due to the ability of the heart to increase blood pressure and blood flow rate in normal operation. Excessive gas as described above substantially reduces such gain. In a close-loop system, the performance of a single component, e.g., the heart, may vary considerably from its curve of normal performance without a great effect upon closed-loop performance as indicated by closed-loop gain. For example, as a first order approximation, for an open-loop gain of 10 for a heart a departure from the curve of component performance will have only one-tenth as much effect on closed-loop performance as it does on open-loop performance, as indicated by the above equation. Although, the variable KG(s) can contain many factors representing the heart, lungs, arteries, veins, and the like--it is the heart that is a major factor with respect to gain--due to the pumping action. With this type of analysis, the amount of improvement expected for congestive heart failure by venting the gas can be estimated. Also, this type of analysis indicates that above normal or (superior) performance, such as for athletes, etc. is difficult to achieve.
This invention relates to those apparatuses and methods of venting to reduce the vapor/liquid ratio and thereby improve the capability of pumping liquid by the heart.